1. Field of the Invention
The present invention is directed to a polymerization catalyst. More particularly, the present invention is directed to a catalyst useful in the polymerization of at least one alpha-olefin.
2. Background of the Prior Art
There is probably no subdivision of catalysis that has been as thoroughly developed as the catalytic polymerization of olefin polymers. Catalysts employed in the polymerization of olefins, especially alpha-olefins, more particularly, ethylene, have been the subject of innumerable patents and technical articles. In recent years, a particularly active sector of this technology has focused upon the catalytic formation of so-called "linear low density polyethylene" (LLDPE).
LLDPE, although employed in similar applications as earlier developed low density polyethylene (LDPE), represents an advance in the art in that the polymerization of LLDPE is far less difficult than is the polymerization of LDPE. That is, whereas LDPE is polymerized under very high pressure, with all the complications attendant therewith, more recently developed LLDPE is polymerized at far lower pressure, simplifying and easing the costs and complications of this reaction. Because LDPE and LLDPE, although chemically distinct, can be utilized in the same applications, this new polymer has rapidly grown in commercial importance.
The development of LLDPE has spurred a parallel development of catalysts useful in its polymerization. Two major goals have been focused upon in evaluating a catalyst useful in the polymerization of LLDPE. The first of these factors is the effect of the catalyst on higher alpha-olefin comonomer incorporation in the LLDPE. As those skilled in the art are aware, LLDPE is a copolymer of ethylene and a higher alpha-olefin, usually a C.sub.4 to C.sub.10 alpha-olefin. A common problem associated with the catalysts of the prior art employed to polymerize LLDPE has been the poor incorporation of the higher alpha-olefin in the final copolymer.
A LLDPE typically incorporates up to about 10 weight percent of a higher alpha-olefin, based on the total weight of the ethylene-higher alpha-olefin copolymer. Although only up to about 10 weight percent of the higher alpha-olefin is included in the copolymer, unfortunately, a much higher concentration of the higher alpha-olefin must be reacted to produce this result. This, of course, results in higher processing expense in that the higher alpha-olefin must be heated and pressurized although it is not polymerized. Thus, an aim of LLDPE catalyst designers continues to be the development of a catalyst which more efficiently incorporates the higher alpha-olefin monomer charged in the polymerization reactor into the copolymer product.
The second major goal by which a catalyst is judged in the polymerization of LLDPE is its hydrogen response. That is, hydrogen is charged into ethylene polymerization reactors to modify the polymer's degree of polymerization. This degree of polymerization is manifested, in the case of an ethylene polymer, by its melt index. If the degree of polymerization is too high, its viscosity is excessive, as defined by a very low melt index. Thus, hydrogen is incorporated in the polymerization reaction to ensure that the degree of polymerization is not excessive. That is, hydrogen is added to guarantee that the LLDPE melt index is sufficiently high. As in the case of higher alpha-olefin incorporation, increasing inclusion of hydrogen increases the cost of polymerization. That is, greater concentrations of unreacted hydrogen result in greater thermodynamic costs of heating and pressurization. Thus, the lesser amounts of hydrogen necessary to produce reasonable melt index LLDPE products result in more attractive polymerization. This result is a function of the polymerization catalyst. Thus, a critical property of a LLDPE catalyst is its so-called "hydrogen response," the ability of the catalyst to efficiently utilize the hydrogen present to modify the degree of polymerization of the LLDPE product.
The development of LLDPE polymerization catalysts has not, in the prior art, reached a point where these desirable properties have been optimized. There are, however, a multiplicity of known catalysts which will be recognized as being similar, in their method of formation, to the catalyst of the present invention.
For example, U.S. Pat. No. 4,252,670 describes an olefin polymerization catalyst formed by treating a magnesium hydrocarbyl, or a complex or mixture of a magnesium hydrocarbyl compound and an aluminum hydrocarbyl compound, with at least one halogenating agent; reacting this product with a Lewis Base, which may be an ether, an ester, a ketone, an alcohol, a thioether, a thioester, a thioketone, a thiol, a sulfone, a sulfonamide or the like; and then reacting the thus formed reaction product with titanium tetrachloride.
U.S. Pat. No. 4,496,660 describes a catalyst for the polymerization of olefins which is initially the reaction product of a hydrocarbyl magnesium compound or a bonded mixture of a hydrocarbyl magnesium and a hydrocarbyl aluminum, zinc or boron, and an oxygen-containing and/or nitrogen-containing compound, such as an alcohol or an amine. This initial reaction product is reacted with a halide source, a halide-containing aluminum, silicon, tin, phosphorus, sulfur, germanium, carboxy, hydrogen, hydrocarbyl or Group IV-B metal, Group V-B metal, Group VI-B metal compound or mixtures thereof. This product, in turn, is reacted with a transition metal compound, which may be titanium tetrachloride, and with a reducing agent, a boron, aluminum, zinc or magnesium organic compound, to form the catalyst.
U.S. Pat. No. 4,295,992 describes an olefin polymerization catalyst formed by the reaction of an aliphatic alcohol with a mixture of an dialkylmagnesium compound and a silicon tetrahalide. This product is then treated with an organic titanium compound, such as titanium tetrachloride, and, finally, with a suitable reducing agent, such as diethylaluminum chloride.
Although the above discussed prior art disclosures advance the art involving the catalytic polymerization of alpha-olefins, none of them, nor any other of innumerable other prior art references, are particularly useful in the polymerization of LLDPE. That is, no polymerization catalyst has been identified which both polymerizes ethylene and is characterized by excellent hydrogen response as well as higher alpha-olefin copolymer incorporation capability.